1. Field Of The Invention
The present invention relates to surgical devices, namely orthopedic bone screws and more particularly to a high torque, low modulus, osteoinductive bone screw article and a method of making said bone screw wherein the screw is polymeric with an interval fiber reinforcement, the fibers being pre-torqued prior to and during a curing or molding with a polymer matrix. The polymer matrix is used to bond with the pre-torqued fibers in the shaft and head regions of the bone screw, and the outer surface of the pre-torqued fiber reinforced polymer screw can be coated with an osteoinductive material such as BMP, growth factors, hydroxyapatite or calcium-phosphate material.
2. General Background
Metal bone screws are routinely used to attach bone plates or to fix fractured bone fragments or tissue to bone. Most commonly used are bone screws made of 316L stainless steel. Stainless steel has an elastic modulus of about thirty (30) million p.s.i. significantly greater than that of cortical bone (about two (2) million p.s.i.). The presence of a hole in cortical bone acts as a stress concentrator and can weaken the bone. For example, Edgerton, et al. (J. Ortho. Res., 8,851-855, 1990) have shown that a hole size equal to twenty (20) percent of the bone diameter can reduce torsional strength by about thirty four percent (34% ).
When steel screws are removed from plated femurs or tibias after fracture healing, the bone often fractures at the open hole remaining upon removal of the screw. Although the tendency for bone fracture may be less with the steel screw in the bone (i.e. filling the hole) the mismatch in elastic modulus between the steel screw and bone can still produce a certain level of stress concentration. A low modulus screw could be left in place and not allow stress shielding of the bone from the presence of the bone plate. The plate (even metal) would maintain acceptable axial and torsional motion at the bone fracture for healing, but because the attachment screws are of low modulus, flexion of the screw can occur allowing load to transfer through the healed bone in a near-normal fashion. Thus a second surgery to remove the device and screws would not be necessary. A bone screw of low modulus poses a smaller stress concentrator in the bone, thus bone integrity would be similar to that of bone without the bone screws present.
To further assure a minimal or nonexistent reduction in bone strength from the presence of the low modulus screw, the screw or other type of anchoring device could be coated or bonded with BMP, growth factors, hydroxyapatite or another effective osteoinductive (osteogenic) material to fully integrate the surrounding bone with the low modulus screw, further reducing any remaining stress concentration effects.
Various patents have been issued which discuss self-reinforced, absorbable materials having reinforcing elements that are at least partially around some axis pertaining to the implant. For example, U.S. Pat. No. 4,968,317 describes a resorbable material in which the reinforcing elements are formed by fibrillating a sheet of the reinforcing material by drawing it.
U.S. Pat. No. 4,743,257 describes a resorbable material in which the reinforcing elements are parallel threads of the same chemical composition as the rest the implant. U.S. Pat. No. 4,743,257 describes an osteosynthesis composite material which is at least partially absorbable in living tissue. This material comprises an absorbable polymer of copolymer matrix which is reinforced with absorbable polymeric reinforcement elements which have the same chemical element percentage composition as does the matrix. The reinforcing element is shown as parallel threads of polymer. International patent application 90/12550 describes a self-reinforced absorbable surgical material characterized in that the reinforcing elements are wound at least partially around some axis penetrating the implant. The spiral orientation of the reinforcing elements is claimed to allow the screws to resist higher torque forces than known parallel thread or non-reinforced screws.
In surgery it is known to use implants, or their parts or components, which are manufactured at least partially of an absorbable polymer and/or of a polymer composite containing reinforcing elements, for fixation of bone fractures, osteotomies or arthrodeses, joint damages, tendon and ligament damages etc. Such implants are e.g. rods, screws, plates, intramedullary nails and clamps, which have been described in the professional literatures of material technique and medicine.
U.S. Pat. No. 3,620,218, entitled "Cylindrical Prosthetic Devices of Polyglycolic Acid" and U.S. Pat. No. 3,739,733 entitled "Polyglycolic Acid Prosthetic Devices", both issued to E. Schmitt and R. Polistina, describe implants like rods, screws, plates, and cylinders which have been manufactured of polyglycolic acid.
U.S. Pat. No. 4,052,988 issued to N. Doddi, C. Versfelt and D. Wasserman, entitled "Synthetic Absorbable Surgical Devices of Polydioxanone" describes absorbable sutures and other surgical devices manufactured of polydioxanone.
U.S. Pat. No. 4,279,249 issued to M. Vert, F. Chabot, J. Leray and P. Christel, entitled "New Prosthesis Parts, Their Preparation and Their Application" describes osteosynthesis devices which have been manufactured of polylactide or of copolymer containing plenty of lactide units, which matrix has been reinforced with reinforcing elements which have been manufactured of polyglycolide or of copolymer including mainly glycolic acid units.
DE 2947985 A 1, issued to S. Belych, A. Davydov, G. Chromov, A. Moscenskij, I. Movsovic, G. Rojtberg, G. Voskresenskij, G. Persin and V. Moskvitin, entitled "Biodestruktiver Stoff fur Verbindungselemente fur Knochengewebe" describes at least partially degradable composites which comprise a copolymer of methylmethacrylate and N-vinlpyrrolidone, which has been reinforced with polyamide fibers or with oxycellulose fibers.
U.S. Pat. No. 4,243,775 issued to M. Rosensaft and R. Webb, entitled "Synthetic Polyester Surgical Articles" describes surgical products manufactured of copolymer of glycolic acid and trimethylene carbonate.
U.S. Pat. No. 4,329,743, issued to H. Alexander, R. Parsons, I. Strauchler and A. Weiss, entitled "Bioabsorbable Composite Tissue Scaffold" describes a composite of a bio-absorbable polymer and carbon fibers, which composite is suitable for manufacturing surgical articles.
U.S. Pat. No. 4,343,931, issued to Barrows, entitled "Synthetic Absorbable Devices of Poly(esteramides)" describes absorbable polyesteramides, which are suitable for manufacturing of surgical implants.
Patent Application EPO 0,146,398, issued to R. Dunn and R. Casper entitled "Method of Producing Biodegradable Prosthesis and Products therefrom" describes a method for manufacturing of biodegradable prosthesis about biodegradable polymer matrix which is reinforced with biodegradable ceramic fibers.
WO 86/00533, issued to J. Leenslag, A. Pennings, R. Veth and H. Jansen entitled "Bone Implant" describes an implant material for reconstructive surgery of bone tissue, which material comprises a biodegradable porous polymer material and biodegradable or biostable fibers.
The publication of D. Tunc, "A High Strength Absorbable Polymer for Internal Bone Fixation" 9th Annual Meeting of the Society for Biomaterials, Birmingham, Alabama, Apr. 27-May 1, 1983, p. 17, describes a high strength absorbable polylactide, with an initial tensile strength about 50-60 MPa and which material retains a significant part of its initial strength 8-12 weeks after the implantation. This material can be considered suitable to be applied as basic material in manufacturing of internal bone fixation devices which are totally absorbable in living tissues.
The publication of D. Tunc, M. Rohovsky, W. Lehman, A. Strogwater and F. Kummer entitled "Evaluation of Body Absorbable Bone Fixation Devices" 31st Annual ORS, Las Vegas, Nevada, Jan. 21-24, 1985, p. 165, describes high strength, totally absorbable polylactide (initial strength 57,1 MPa), which was used as plates and screws for fixation of canine radial osteotomies.
The publication of D. Tunc, M. Rohovsky, J. Zadwadsky, J. Speiker and E. Strauss entitled "Evaluation of Body Absorbable Screw in Avulsion Type Fractures", the 12th Annual Meeting of the Society for Biomaterials, Minneapolis-St. Paul, Minn., USA, May 29 to Jun. 1, 1986, p. 168, describes the application of high strength polylactide screws in fixation of avulsion-type fractures (fixation of canine calcaneus osteotomy).
U.S. Pat. No. 4,776,329 issued to R. Treharne entitled "Resorbable Compressing Screw and Method", describes a compression screw comprising a non-absorbable compression parts and a screw. At least the head of the screw comprises material, which is resorbable in contact with tissue fluids.
Self-reinforced absorbable fixation devices have significantly higher strength values than the non-reinforced absorbable fixation devices. U.S. Pat. No. 4,743,257 issued to P. Tormala, P. Rokkanen, J. Laiho, M. Tamminmaki and S. Vainionpaa entitled "Material for Osteosynthesis Devices" describes a self-reinforced surgical composite material, which comprises an absorbable polymer or copolymer, which has been reinforced with absorbable reinforcing elements, which have the same chemical element composition as the matrix.
FI Patent Application No. 87 0111, issued to P. Tormala, P. Rokkanen, S. Vainionpaa, J. Laiho, V.-P. Heponen and T. Pohjonen entitled "Surgical Materials and Devices" describes self-reinforced surgical bone fracture fixation devices which have been manufactured at least partially of fibrillated absorbable material(s).
According to the publication of T. Pohjonen, P. Tormala, J. Mikkola, J. Laiho, P. Helevirta, H. Lahde, S. Vainionpaa and P. Rokkanen entitled "Studies on Mechanical Properties of Totally Biodegradable Polymeric Rods for Fixation of Bone Fractures" VIth International Conference PIMS, Leeuwenhorst Congress Center, Holland, Apr. 12-14, 1989, p. 34/1-34/6, self-reinforced absorbable surgical materials have excellent strength properties, e.g. SR-polyglycolide had bending strength 415 MPa and SR-polylactide 300 MPa.
In the publication of D. Tunc and J. Jadhav entitled "Development of Absorbable Ultra High Strength Polylactide", Am. Chem, Soc., 196th ACS Meeting, Abstracts of Papers, L.A., Calif., Sep. 25-30, 1988, p. 383-387, a good tensile strength (300 MPa) for fibrillated SR-polylactide was measured.
The publication of E. Partio, O. Bostman, S. Vainionpaa, H. Patiala, E. Hirvensalo, K. Vihtonen, P. Tormala and P. Rokkanen entitled "The Treatment of Cancellous Bone Fractures with Biodegradable Screws", Acta Orthop. Scand., 59(5), 1988, p. 18, describes the fixation of cancellous bone fractures with self-reinforced absorbable screws, which have a flat head, which head can be located to a slot at the tip of the screwdriver in order to drive the screw into a channel made into the bone.
The following patents relate to absorbable (biodegradable or resorbable) polymers, copolymers, polymer mixtures, or composites: U.S. Pat. No. 3,297,033; U.S. Pat. No. 3,636,956,U.S. Pat. No. 4,052,988; U.S. Pat. No. 4,343,931; U.S. Pat. No. 3,969,152; U.S. Pat. No. 4,243,775; FI Patent Appln. No. 85 5079, FI Pat. Appln. No. 86 0366; FI Patent Appln. No. 86 0440 and FI Pat. Appln. No. 88 5164.